In recent years, considerable efforts have been made by the coatings industry to develop coating formulations containing little or no volatile organic compound (VOC) content. Regulations to limit the VOC content of industrial coatings have encouraged research and development to explore new technologies directed at reducing solvent emissions from industrial solvent-based coatings operations for such products as automobiles, appliances, general metal products, furniture, and the like. However, while the move to reduced organic solvent-based compositions brings health and safety benefits, these lower VOC coating compositions must still meet or exceed the performance standards expected from solvent-based compositions.
Alkyd resins are one of the most common binders used for ambient-cure, solvent-based coatings. An alkyd is typically prepared by reacting a diol, a polyol, a polyacid, a monofunctional acid, and a fatty acid, fatty ester, or a naturally occurring, partially-saponified oil, optionally in the presence of a catalyst. More specifically, an alkyd resin can be the reaction product of: (i) from 0 to about 30 mol % of a diol, (ii) from about 10 to about 40 mol % of a polyol, (iii) from about 20 to about 40 mol % of a polyacid, (iv) from 0 to about 10 mol % of a monofunctional acid, (v) from about 10 to about 50 mol % of a fatty acid, fatty ester, or naturally occurring oil, and optionally, (vi) a catalyst, wherein the mole percents are based on the total moles of (i), (ii), (iii), (iv), (v), and (vi), if present. Suitable examples of each of the components of the alkyd resin include those known in the art, including, but not limited to, those discussed below, and in Resins for Surface Coatings, Vol. 1, p. 127, ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, incorporated herein by reference.
The resistance properties of traditional solvent-borne alkyd resins are a result of autooxidative crosslinking of the alkyd film upon application to a substrate. Crosslinking occurs when the activated methylene groups in the unsaturated fatty acids or oils of the alkyd are oxidized in air to give hydroperoxides, which subsequently decompose to generate free radicals, leading to oxidative crosslinking. This oxidative crosslinking process is commonly accelerated by adding driers, such as, for example, the various salts of cobalt, zirconium, calcium, and manganese. However, while alkyd resins have shown, and continue to show, promise, they have relatively slow “dry” and/or cure times, particularly at ambient temperatures.
Conventional long oil alkyds are nonetheless used throughout the coatings industry as the main binder in high gloss architectural trim enamels. Typical alkyds are made from (i) soybean oil, reacted with pentaerythritol (PE) via alcoholysis, and then (ii) reacted in a second stage with phthalic anhydride (PAN). The result is a long oil alkyd with good through-dry (the PE allows an alkyd with high branching and number average molecular weight [Mn]), light color, yellowing resistance, and low cost. However, these conventional long oil alkyds require a large amount of organic solvent (>350 g/L VOC) for use in paint formulations. These resins typically have an acid number of from 2 to 10, a relatively low acid number which is desirable to ensure complete polycondensation to yield alkyd resins having reasonably high molecular weight. A relatively high molecular weight, in turn, leads to more acceptable drying times for the coatings.
High-solids alkyds have been developed for use in 250 g/L VOC, high gloss architectural trim enamels. Reduction in viscosity in these resins is achieved by lowering the amount of PE, which results in less branching and a lower Mn. One such alkyd is Duramac HS 5816, available from Eastman Chemical Company, which is made from (i) sunflower oil reacted with pentaerythritol (PE) via alcoholysis, followed by (ii) a fatty acid, and (iii) phthalic anhydride. The result is a long oil alkyd having a reasonable through-dry and a light color at a reasonable cost, but with reduced yellowing resistance.
There is, then, a trade-off between through-dry and yellowing. Less yellowing is observed with the use of less conjugated fatty acids and oils, but with the disadvantage that the through-dry properties are adversely affected. An additional drawback with high-solids alkyds is that typical high-solids alkyds result in paints that exhibit stringiness or ropiness (brush drag and high ICI viscosity).
Various modifications have been made to alkyd resins in an attempt to address these concerns. One such attempt involves polymerization of an alkyd resin with a vinyl compound, such as styrene or methyl methacrylate, via a free-radical reaction, to produce a vinyl-alkyd copolymer or a vinyl alkyd. Vinyl alkyd resins generally have a higher molecular weight and a higher Tg, producing coatings with reduced tack-free time (solvent evaporation). However, the through-dry time (oxidation of the film) of such coatings is longer due to the decreased degree of unsaturation in the alkyd resulting from the copolymerization with the vinyl compound. This problem is described in further detail in Resins for Surface Coatings, Vol. 1, p. 181, ed. by P. K. T. Oldring and G. Hayward, SITA Technology, London, UK, 1987, which is incorporated herein by reference. An additional drawback with vinyl alkyd resins is that paint formulations containing vinyl alkyd resins require a higher content of organic solvent, due to the increased molecular weight and Tg of the vinyl alkyd.
JP 48085628 (hereinafter JP '628) describes light-curable coating compositions made from a drying oil-modified alkyd resin which is further modified using glycidyl acrylate, glycidyl methacrylate, or its derivative. In this reference, drying oil-modified alkyd resins having —CO2H groups and an oil length of 20-80 are treated with glycidyl acrylate, glycidyl methacrylate, or its derivative, in the presence of a polymerization inhibitor. In a specific embodiment, a drying oil-modified alkyd resin having an acid number of 100 and an oil length of 34 is reacted with 36 parts glycidyl methacrylate, to give a resin varnish having an acid number of 5.0.
Both the acid number of the drying oil-modified alkyd resin of JP '628, and the amount of glycidyl methacrylate used, are relatively high, requiring the use in the reaction mixture of hydroquinone, a polymerization inhibitor, to prevent the alkyd from gelling during resin synthesis. One drawback of this approach is that the presence of a polymerization inhibitor in paint formulations is known to prolong the drying times of the resulting coating films. Moreover, the disclosed alkyd resin composition of JP '628 contains an amine monomer, triethanolamine, which is desirable for the UV cure application intended, but can cause detrimental effects on oxidative cure. The resin in JP '628 is afterward mixed with a photosensitizer or a photoinitiator to give a coating composition which hardens with UV irradiation. Accordingly, the disclosed coating composition requires the use of a photosensitizer or photoinitiator, and UV irradiation, in order to carry out the teaching. The reference does not teach a coating composition suitable for ambient oxidative cure, high-solids coating applications.
PCT Publ. No. WO 01/00741 A1 discloses an ambient oxidative cure composition based on an acrylate-functionalized alkyd resin. The acrylate-functionalized resin is prepared by reacting a hydroxyl-functional alkyd resin, for example with an acid number of from 0 to about 10 mg KOH/g, with about 2-8 mole % of an acid anhydride, such as trimellitic anhydride, to produce a carboxyl-functional alkyd resin. The carboxyl-functional alkyd resin is thereafter reacted with a glycidyl acrylate, to produce an acrylate-functionalized alkyd resin having an acid number of less than about 5. A disadvantage of the process described is that the resin is first carboxylated with an acid anhydride, such as trimellitic anhydride, to increase the acid number, prior to functionalizing the resin with a glycidyl acrylate. It would be advantageous to dispense with this carboxylation step, and to directly react the alkyd with the glycidyl acrylate, while still obtaining a coating which exhibits an acceptable dry time.